(1) Field of the Invention
This invention relates to additives to hydraulic cements. In a particular aspect, it relates to additives for use in improving the grinding and set retarding characteristics of hydraulic cements, to the process for making these cements and to the resultant hydraulic cements containing these additives.
(2) Description of the Prior Art
Hydraulic cements are produced by calcining suitable raw materials, generally a mixture of calcareous and argillaceous materials, to produce a sintered "clinker." Of the hydraulic cements, portland types are by far the most important cements in terms of quantity produced. Portland cement represents a class of hydraulic cements and is comprised essentially of two calcium silicates and a lesser amount of calcium aluminate. These cements are produced by heating an intimate mixture of finely divided calcareous material (limestone) and argillaceous material (clay) to fusion to form a clinker. The clinker is ground with the addition of about 3% to 3.5% of gypsum, or some other form of calcium sulfite, to obtain desired setting qualities in the finished cement. Small amounts of other substances are sometimes added during grinding to impart special properties to the product.
Grinding of the clinker consumes substantial quantities of time and energy. It is common practice in the cement industry to employ grinding aids which increase the efficiency of the grinding operation thereby lowering the power required to grind a unit of cement and which decrease the plant investment either by increasing the through put of a given grinding mill or by reducing the size of mill needed to grind a given amount of cement. The addition of a grinding aid enables the mill to grind the clinker to a smaller size with less energy by prohibiting the buildup of a coating of finer material on the grinding media and walls of the mill by coating the nascent surfaces of the cement clinker. This coating of the cement clinker reduces the higher energy forces present on the nascent surface. Caution must be taken to prevent undue reduction of the surface forces because the cement becomes excessively fluid.
Known grinding aids include a water-soluble glycol and a water-soluble alkali metal salt of an aliphatic acid having no more than three carbons, which composition also acts as a pack set inhibitor (U.S. Pat. No. 3,615,785). U.S.S.R. Pat. No. 566,788 discloses the still residues of higher aliphatic acid salts and polyethylene glycol monoalkylphenyl ethers as additives to increase the effectiveness of pulverization of a cement clinker. Also, polypropylene glycol and/or polyglycol are disclosed as components of an additive composition containing triethanolamine for use as a cement grinding aid in South African Pat. No. 74 03,631.
It was only when the appreciable effect of cement fineness on the rate of its strength development was discovered, and when the fineness was increased that cases of flash setting became more frequent. The discovery that a small addition of gypsum can overcome this trouble was a considerable step forward in cement technology.
However, even with the inclusion of gypsum, an early or premature stiffening (false set) sometimes occurs, which can be troublesome if it occurs before the concrete is placed. False set in cement is evidenced in concrete by a significant loss of consistency shortly after mixing. After additional remixing, however, this concrete regains its original consistency.
Also, false set notwithstanding, in certain concrete applications a longer time is required for proper emplacement in the formation in the case of building construction concrete or, in masonry cement, a longer "board life" is needed, and a longer setting time than allowed by gypsum alone is necessary for an acceptable safety margin to do a satisfactory job.
Known set retarders for cement include: D-glycero-D-guloheptonolactone (U.S. Pat. No. 3,686,008); a salt of lignosulfonic acid combined with boric acid or a borate (U.S. Pat. No. 3,662,830); an alkanol-amino-lignosulfate-formaldehyde (U.S. Pat. No. 3,689,296); and, carbohydrates (mono-, di-, tri- and polysaccharides as well as derivatives thereof), polyhydroxy polycarboxylic compounds, lignosulfonic acid and salts thereof, water-soluble salts of boric acid, zinc oxide, and water-soluble silicone compounds and mixtures of these (U.S. Pat. No. 3,865,601).
U.S. Pat. No. 4,089,696 teaches the addition to a portland type cement mix of an acetic acid ester of glycerin to accelerate the rate of hardening of the mix and to increase the compressive strength of the mix when hardened.
It is the primary object of this invention to provide an additive for hydraulic cements which function as both grinding aids and set retarders.
It is a further object of this invention to provide a hydraulic cement containing an additive which functions both as a grinding aid and set retarder.
It is another object of this invention to provide a process for increasing the grinding efficiency of and retarding the hardening of hydraulic cement by the addition thereto of a small but effective amount of an additive without deleteriously affecting the desirable properties of the cement.
The object of set retardation may be achieved by adding the set retarder of the invention to the hydraulic cement either during grinding or in solution along with the water mixture of the ground cement.